The present invention relates to the preparation of ion-conducting, solvent-free solid polymeric systems characterized as being cationic single-ion conductors. Solvent-free polymer electrolytes have generated significant interest in recent years due to their versatility. For instance, solvent-free solid polymer electrolytes have been used in high energy density batteries, photoelectrochemical cells and solid state electrochromic displays.
Like liquid electrolytes and solvent-swollen polyelectrolytes used in ion-exchange resins, solvent-free polymer electrolytes possess ion-transport properties. Both cation transport and anion transport in these solid polymer electrolytes have been substantiated and are well documented in the prior art. However, these solid polymer electrolytes exhibit far lower ionic conductivity than either the liquid or solvent swollen (poly)electrolytes.
It is believed that the lower conductivity of solid polymer electrolytes is caused by extensive ion pairing and ion clustering which, in effect, ionically crosslinks the polymer. The resulting solid electrolyte is brittle and glassy at room temperature. It is known to use plasticizers in polymers to increase polymer chain flexibility by reducing intermolecular attractions, to increase free volume and to decrease the glass transition temperature, Tg. These changes are also known to increase ionic conductivity in solid polymer electrolytes.
Early investigation in the solvent-free polymer electrolyte field focused on alkali metal ion conduction in solid electrolytes formed by alkaline metal salts and poly(ethylene oxide) or poly(propylene oxide). See, for example, M. B. Armand, Proc. on the Workshop on Lithium Nonaqueous Battery ElectroChemistry, Cleveland, June 1980, pp. 261-270. However, substantial anion mobility was found in electrolytes made from a charged polymer salt complex. When such a complex is employed in a battery, for instance a lithium battery, it produces a negative effect on the energy efficiency of the battery because it results in local concentration gradients which result in deleterious polarization of the cell, lowering the output current.
Attempts have been made to immobilize the anion on the polymer chain in order to achieve specific cationic activity. Several approaches have included the synthesis of cationic single-ionic conductors based on carboxylate or sulfonate salts. These reported electrolytes are limited in their application due to low conductivity or lack of electrochemical stability. Presumably, the low conductivity is due to the extensive ion-pairing in theses salts.
A more recent approach to overcome the extensive ion pairing in these salts is described in U.S. Pat. No. 4,471,037 to Bannister. In that patent, Bannister describes a lithium-polyether complex which is prepared by employing an atactic polyether having a glass transition temperature of substantially less than 0.degree. C. and being capable of forming a complex with Li.sup.+ ions but not with Na.sup.+ ions. Because the polyether is atactic, that is, the polyether has no stereoregularity, it has an enhanced proportion of amorphous regions. Bannister theorizes that high ionic conductivity may occur via such amorphous regions. Moreover, Bannister suggests that because the polyether is not capable of forming complexes with Na.sup.+, the Li.sup.+ ions in the electrolyte are more loosely bound to the polyether, thereby giving rise to higher lithium ion conductivity. While the Bannister complex represents an advance in the art, it represents a limited advance insofar as this complex can be prepared only from atactic polyethers and lithium salts.